The present invention relates to the art of data verification. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the invention may also find application in other types of data acquisition and handling processes, including magnetic resonance spectroscopy, medical diagnostic imaging, and the like.
Heretofore, patient motion has been a major cause of image degradation in magnetic resonance imaging. Patient motion causes streaking or ghosting in the phase encode direction, loss of resolution, blurring, and the like. Ghosting in the phase encoding direction is often attributable to periodic errors or motion; whereas, streaking is often attributable to random errors.
In one common magnetic resonance imaging technique, a magnetic resonance excitation or 90.degree. RF pulse is applied to excite magnetic resonance of selected dipoles in an image slice or region. An additional pulse or pulses, such as a 180.degree. RF inversion pulse, is applied to induce the resonating nuclei to converge and form a magnetic resonance echo. Phase encode and read gradient pulses are applied for phase and frequency encoding the magnetic resonance data along orthogonal phase encode and frequency encode axes or directions in the selected slice region. During each magnetic resonance echo, resonance data are collected and digitized to form one view or line of magnetic resonance data. The sequence is repeated with each of a plurality of different phase encode gradients. The set of views with one corresponding to each of the plurality of phase encodings is Fourier transformed to form an image representation.
The artifacts are attributable to the difference between a view collected during motion and the same phase encoded view collected in the absence of motion. If the subject moves to a different location in the frequency encode direction, each view has different frequency components. Physical displacement and motion in the direction of the phase encode gradient both result in changes of the phase of the acquired data values of the resultant view. Further, the movement of non-saturated spins into or out of the image slice causes changes in amplitude of the data values of the resultant view.
As an object moves through magnetic field gradients, its resonant dipoles accumulate phase shifts which are directly related to associated orders of motion, e.g. velocity, acceleration, pulsatility, or the like. If the nature of the motion changes from view to view, even although the physical position remains constant at the time of data collection, the phase of the resultant view will change. Also, the amplitude, phase, and frequency spectrum of the resultant view are altered by motion into or out of the image slice. As partially saturated material in the slice is replaced by non-saturated material from out of the slice, amplitudes increase. In multi-slice imaging, partially saturated material may move from slice to slice causing an amplitude increase or decrease in a given slice. As an object moves relative to a radio frequency coil, the coil loading and the coil tuning may change. This alters the sensitivity of the receiver and changes the effective flip angles. Motion toward or away from a surface coil changes the intensity of the signal which is received, even without net changes in coil tuning. Any or all of these mechanisms may contribute to data errors and hence, to motion artifacts.
The prior art techniques for reducing or eliminating the effects of motion can be divided into three categories--gating, modified pulse sequences, and modified data collection techniques. In the prior art gating techniques, an external monitor monitored the patient's motion. The collection of data or the application of pulse sequences was blocked when an unacceptable degree of motion or displacement was detected. Analogously, a state of motion could be frozen by triggering a pulse sequence in response to monitoring a preselected state of motion. One of the problems with the prior art gating techniques is that the monitors commonly monitored only physical position. A moving body part, even although nominally in the correct location during data collection, still causes phase errors in the resultant view and also a loss of resolution.
In the modified pulse sequences, motion artifacts were reduced by reducing the sensitivity of the pulse sequence to the effects of motion. Although the modified pulse sequences tended to reduce phase errors, the motion related loss of resolution was not recovered and physical displacement or shifting of the patient to a new stationary location was not detected.
The modified data collection schemes commonly included rearranging the order in which views are collected. The views were collected such that the view number corresponded to a selected location, e.g. in respitory applications, the chest wall position. This rearranging reduces periodic motion-induced ghosts. However, modified data collections schemes neither corrected for the motion nor recovered the lost resolution.
The present invention provides for a new and improved data collection technique in which the collected data is examined for evidence of subject movement during the data collection period.